Imaging procedures in the presence of a NAST are:
Dopplersonography: Doppler sonography should be used as a screening procedure. Herewith can be determined:
- Peak systolic velocity (PSV)
- End diastolic velocity (EDV)
- reno- aortic quotient
- Resistive index (RI)
However, false negative results are found in 10 % - 20 %. (Manski 2019)
Duplex sonography: Duplex sonography is most commonly used for diagnosis. It is inexpensive, but the result depends on the examiner. It can be used to visualize the renal arteries, determine the flow velocity, estimate the degree of stenosis (Kasper 2015) and assess the intima of the vessels.
The sensitivity is 88%, the specificity 89% (Kuhlmann 2015).
Suspicious for stenosis are:
- Vmax A. renalis ≥ 2 m / sec
- RI (intrarenal resistance index) < 0.5
- RI- lateral difference > 5 (Herold 2021)
- reno- aortic quotient (RAQ) - assuming a normal flow in the aorta - of > 3.5 (Manski 2019).
However, duplex sonography is less suitable for the diagnosis of fibromuscular dysplasia and any changes in accessory renal vessels (Kasper 2015).
MR angiography: This allows good visualization of the renal arteries and the perirenal branches of the aorta, but not existing stents (Kasper 2015). The sensitivity is 97 %, the specificity 95 % (Kuhlmann 2015).
The examination is costly and should not be used above a GFR of < 30 ml / min / 1.73 m², as gadolinium can lead to nephrogenic systemic fibrosis [Herold 2021]). (Kasper 2015)
CT angiography: This also provides good imaging of the renal arteries and the perirenal branches of the aorta (kasper 2015). The sensitivity is 98% and the specificity is 94% (Kuhlmann 2015). The examination is expensive and a certain amount of contrast medium is required, which can be potentially nephrotoxic (Kasper 2015).
Captopril renal scintigraphy: One week before performing captopril renal scintigraphy, ACE inhibitors must first be paused. Scintigraphy is performed before and 1 h after captopril administration (recommended dosage: 25 mg p. o.). Pathological findings are:
- Decreased unilateral or bilateral function compared to baseline.
- differences in kidney size (signs of organ atrophy)
- delay in maximum secretion (> 11 min)
- the retention of the radionuclide is cortical
The sensitivity and specificity of the test are 90% (Manski 2019).
Intra-arterial angiography: Confirmation of the diagnosis is possible with intra-arterial digital subtraction angiography (i. a.- DSA). However, this examination should only be performed if the patient agrees to possible PTA and the possibility of balloon catheter dilatation exists (Herold 2021).
With i. a. DSA, the severity and exact localization of vascular lesions can be visualized. In diseases of the great vessels, the examination is considered the gold standard and is usually performed during a planned intervention (Kasper 2015).
Dangers of DSA are: athero-embolism, dissection and contrast agent toxicity (Kasper 2015).
The sensitivity of the method is 100%, the specificity is also 100% (Kuhlmann 2015).